The invention relates to receiving spread spectrum radio signals, such as digitally modulated signals in a Code Division Multiple Access (CDMA) mobile radio telephone system, and more particularly, to configuring a RAKE receiver.
In a conventional RAKE receiver, a searcher provides a set of paths to the fingers and diversity combiner of the RAKE receiver. The searcher uses a matched filter (or a similar correlation scheme) to select N paths, where N is the number of fingers. The diversity combiner then allots different weights to each of the N fingers.
Generally speaking, new paths are born and other paths die as a mobile station moves through its environment. If two or more paths die together, it is difficult for the receiver to get enough signal power. As the correlated paths die, it is usually necessary to use the searcher (or matched filter) to find new paths. In some cases, the RAKE receiver has to run the matched filter continuously. Using a matched filter is costly and computationally complex. It is not only time-consuming; it also decreases the battery life of hand-held units.
FIG. 1 is a schematic diagram of an example of a CDMA system. A transmitter 30 can transmit input user data to multiple users. In a traditional CDMA system, each symbol of input user data 31 is multiplied by a short code or chip sequence 33. There is a unique short code for each input user. Input user data is then spread by a long code or chip sequence 35. While the short codes eliminate multiple access interference among users in the same cell, the long code is used to eliminate multiple access interference among the transmitters. An accumulator 36 adds the spread signals to form a composite signal 37. Composite signal 37 is used to modulate a radio frequency carrier 38 which is transmitted by a transmitting antenna 39.
A receiver 50 has a receiving antenna 59 for receiving signal 40. Receiver 50 uses a carrier signal 58 to demodulate signal 40 and to obtain composite signal 57. Composite signal 57 is multiplied by a synchronized long code or chip sequence 55. Long code 55 is a locally generated complex conjugated replica of long code 35.
The despread signal 54 is then multiplied by a synchronized short code or chip sequence. Short code 53 is a locally generated complex conjugated replica of short code 33 (or one of the other N short codes used by transmitter 30). The multiplication by short code 53 suppresses the interference due to transmission to the other users. A digital logic circuit 52 (e.g., a summation and dump unit) can be used to provide an estimate of input user data 31.
It will be evident to those skilled in the art that receiver 50 can not reconstruct input user data 31 unless it can (1) determine long code 35 and synchronize a locally generated complex conjugated replica of long code 35 with the received signal 57, and (2) determine short code 33 and synchronize a locally generated complex conjugated replica of short code 33 with the despread signal 54. It is for this reason that many CDMA signals contain a pilot signal or a periodic code (synchronization code). The synchronization codes can be found by using a matched filter or a correlation scheme and by identifying the correlation peaks.
In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion (multipath delay). Multipath delay is caused by, for example, signal reflections from large buildings or nearby mountain ranges. The obstructions cause the signal to proceed to the receiver along not one, but many paths. The receiver receives a composite signal of multiple versions of the transmitted signal that have propagated along different paths (referred to as “rays”). The rays have different and randomly varying delays and amplitudes.
Each distinguishable “ray” has a certain relative time of arrival, Tn seconds. A receiver can determine the relative time of arrival of each ray by using a matched filter, a shifted search finger, or another correlation scheme. The output of the matched filter or the correlation scheme is usually referred to as the multipath profile (or the delay profile). Because the received signal contains multiple versions of the same signal, the delay profile contains more than one spike.
FIG. 2 is an example of a multipath profile. The ray that propagates along the shortest path arrives at time To with amplitude A0 and phase φ0, and rays propagating along longer paths arrive at times T1, T2, . . . , T30 with amplitudes A1, A2, . . . , A30 and phase φ1, φ2 . . . , φ30, respectively. In order to optimally detect the transmitted signal, the spikes must be combined in an appropriate way. This is usually done by a RAKE receiver, which is so named because it “rakes” different paths together. A RAKE receiver uses a form of diversity combining to collect the signal energy from the various received signal paths (or rays). The term “diversity” refers to the fact that a RAKE receiver uses redundant communication channels so that when some channels fade, communication is still possible over non-fading channels. A CDMA RAKE receiver combats fading by identifying the delay for each path individually and then adding them together coherently.
FIG. 3 is a schematic diagram of a RAKE receiver with N fingers. A radio frequency (RF) receiver 110 demodulates a received signal and quantizes the demodulated signal to provide input signal 112. Each finger uses input signal 112 to recover signal power from a different path. The receiver can use a searcher to find a set of signal paths.
Using the example in FIG. 2, the searcher determines that the peak at T20 has the greatest amplitude. Because this path is the strongest path, one of the fingers, for example, finger 320 is configured to receive a path having a delay of T20. The receiver can be configured by, for example, delaying digital samples 112 by T20 or by shifting chip sequence(s) 321 by an equivalent amount.
Similarly, input signal 112 can be correlated in finger 322 with a chip sequence 323 that has a phase corresponding to T10; in finger 330 with a chip sequence 331 that has a phase corresponding to T5; and in finger 332 with chip sequence(s) 333 having a phase corresponding to T15. The finger outputs are multiplied by individual weights 340, 342, 350, and 352 to maximize the received signal-to-noise-and-interference ratio. The weighted outputs are then added by an accumulator 362. The output of the accumulator 362 is fed to a threshold device 364, or to a quantizer that outputs soft information.
It is important that the RAKE receiver use the best set of paths. However, using a matched filter to search for new paths is costly and computationally complex. There is a need for a diversity scheme that can reduce the computational complexity of the RAKE receiver.